1. Field of the Invention
The present invention relates to a light emitting element using an organic compound at least in part thereof. Moreover, the present invention relates to a light emitting device and an electronic device provided with the light emitting element.
2. Description of the Related Art
A light emitting device using a light emitting element, which has a layer including an organic material between a pair of electrodes and emits light by feeding current between the electrodes, has been developed. Such a light emitting device has advantages in a thin shape, lightweight, favorable visibility due to self-light emission, and fast response compared with another display device which is referred to as a thin display device. Therefore, such a light emitting device has been actively developed as a next-generation display device and is partly put into practical use at present.
Such a light emitting device which is advantageous to be a thin shape and lightweight is especially suitable for application to a mobile device. The lower power consumption of a light emitting device mounted to a mobile device in which battery is limited, the better, and power saving is always requested. In addition, a requirement of reducing energy consumption is more and more increased in association with an environmental problem, an energy problem, and the like in a television, a display, and the like in addition to a mobile device.
By the way, one of the reasons that such a light emitting device is limited to a partial practical use is deterioration of a light emitting element. A light emitting element is deteriorated such that luminance is lowered in accordance with the accumulation of driving time even if the same amount of current is fed. One of the reasons is that a material and a structure for manufacturing a light emitting element in which the degree of the deterioration is acceptable as an actual product has not been so varied yet.
Various methods for reducing deterioration of a light emitting element are conceivable, and one of the methods is to improve luminous efficiency. By the improvement of luminous efficiency, the same luminance can be obtained even if the amount of current to be fed to a light emitting element is small, which results in reduction in deterioration of the light emitting element.
If luminous efficiency is improved, the same luminance can be obtained with lower power, which leads to a low drive voltage and low power consumption.
For example, in Patent Document 1 (Patent Document 1: Japanese Patent Laid-Open No. 2005-108730), the improvement of luminous efficiency is attempted by controlling concentration of dopant in a light emitting layer in a thickness direction of the light emitting layer.